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Originally Posted by MeoTwister5
First and foremost to call a something a "biological defect" is completely subject in reference to climate, environment, relative effects on biological systems etc. There is no such thing as an absolute defect because what is defective today may be an advantage a hundred years from now.
To expound of Xellos' explanation on Sickle Cell Anemia, it is actually a very good representation of the concept of how a genetic defect may be an advantage. HbS has a glutamate substitution for valine at position 6 of the Beta-globulin chain due to single point mutations of the gene, which results in hemoglobin to form rigid polymer structures rather than its loose conformation that allows for better. At low oxygen saturation, the molecule aggregates and precipitates, causing the sickling of RBCs that both cause tearing of the cell and shearing of the vascular walls on close contact.
Malarial species are erythrocytic for reproduction inside the human host. They require a metabolically working RBC to be able to properly form gametocytes. HbS and other RBC diseases such as G6PD and the forms of Thalassemia prevents proper gametocyte formation, giving partialimmunity to Malarial diseases. It's even been posited that it's possible certain forms of RBC mutations may have even been induced as a response against Malarial infection.
Might as well add that there are currently no nanoviridae known to infect humans. The smallest appreciable human infecting virus is parvovirus B19. nanoviridae are exclusively plant-infecting.
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Still sickle cell anemia is more common in Eastern European countries, noticibly in the nobility during the middle ages. Of all places to develop sickle cell anemia is not a evolutionary response to malaria. It really depends on how these "defects" occur, but sickle cell anemia is still detrimental due to the lack of clotting factors nad the need of platelets to clot.